Why Selenium Deficiency Is So Common in UK Horses

Why Selenium Deficiency Is So Common in UK Horses

If you've ever had your horse's blood tested and been told their selenium levels are low, you're far from alone. Selenium deficiency is one of the most common mineral imbalances seen in UK horses — and there are very specific reasons why horses on this side of the Atlantic are particularly vulnerable.

In this article, we'll look at why selenium deficiency is so widespread in the UK, what it does to your horse's health, how to recognise the signs, and — crucially — how to address it safely without tipping into toxicity.

What Is Selenium and Why Does Your Horse Need It?

Selenium is a trace mineral — meaning your horse only needs it in tiny amounts, but those tiny amounts are absolutely essential. It plays a critical role in several key body functions:

• Antioxidant defence: Selenium is a core component of glutathione peroxidase, an enzyme that protects cells from oxidative damage. It works hand in hand with vitamin E.
• Immune function: Horses with adequate selenium levels mount stronger immune responses to infection and vaccination.
• Muscle health: Selenium is vital for normal muscle function, including the heart muscle.
• Thyroid metabolism: Selenium-dependent enzymes are involved in converting thyroid hormones into their active forms.
• Reproductive health: In breeding stock, selenium influences fertility, foal development, and colostrum quality.

The recommended daily selenium intake for an average 500kg horse is around 1–2mg per day. That sounds almost negligible — but getting even that small amount from a UK-based diet can be surprisingly difficult.

The Root Cause: UK Soils Are Naturally Low in Selenium

The single biggest reason selenium deficiency is so common in UK horses comes down to geology. The soils across most of Britain and Ireland are naturally low in selenium.

Selenium levels in soil vary enormously around the world. Parts of the United States, particularly the Great Plains states, have selenium-rich soils — sometimes so rich that selenium toxicity in livestock is a genuine concern. In contrast, much of Western Europe, Scandinavia, and especially the UK sits on soils that are selenium-poor.

Why Are UK Soils So Low?

Several factors contribute:

• Underlying geology: Much of the UK sits on sedimentary rocks and acidic soils that are inherently low in selenium.
• Soil pH: Acidic soils (common across much of the UK, particularly in the north and west) bind selenium into forms that plants cannot absorb effectively.
• Rainfall: The UK's high rainfall leaches water-soluble selenium compounds out of the topsoil and into waterways.
• Soil type: Heavy clay soils, prevalent in many parts of England, tend to lock selenium into unavailable forms.

The result is that the grass, hay, and haylage produced on UK land is almost always low in selenium. Research consistently shows that UK-grown forages typically contain well below 0.1mg/kg of selenium on a dry matter basis — often closer to 0.02–0.05mg/kg. To put that in context, a horse eating 10kg of dry matter per day from such forage would be getting somewhere between 0.2mg and 0.5mg of selenium — well below the 1–2mg recommended daily intake.

The Forage Problem: Your Horse's Main Feed Is the Main Gap

Forage — grass, hay, and haylage — makes up the vast majority of most horses' diets, typically 70–100% of their total dry matter intake. This is exactly as it should be for digestive health. But when it comes to selenium, it means the bulk of the diet is contributing very little.

Unlike some other minerals where forage might provide a reasonable baseline that just needs topping up, selenium from UK forage is so low that horses are almost entirely dependent on supplementary sources.

What About Hard Feed?

Commercial compound feeds (cubes, mixes, and balancers) are typically fortified with selenium, and this is one of the main ways UK horses receive the mineral. However, there are some common scenarios where this safety net fails:

• Horses on forage-only diets: Good doers, native breeds, and many leisure horses thrive on hay or grass alone, with no compound feed. Without a balancer or supplement, they get almost no supplementary selenium.
• Under-feeding compound feed: Many owners feed below the manufacturer's recommended rate. If a feed is designed to be fed at 3kg per day but is being fed at 1kg, the horse is only receiving a third of the intended selenium.
• Feeding straights: Owners who feed plain oats, barley, or unmolassed beet without a balancer are adding calories without adding meaningful selenium.

This is why analysing your horse's diet is so valuable. Without looking at the numbers, it's very easy to assume your horse's mineral needs are being met when they simply aren't.

Signs of Selenium Deficiency in Horses

Selenium deficiency can be subtle and insidious. Because the mineral is involved in so many body systems, the signs can be vague and easily attributed to other causes.

Common Signs Include:

• Poor muscle condition or stiffness: This is one of the classic signs. In severe cases, selenium deficiency can cause white muscle disease (nutritional myodegeneration), particularly in foals.
• Reduced exercise tolerance: Horses may fatigue more easily or recover slowly after work.
• Weak immune function: Frequent infections, slow wound healing, or poor response to vaccination can all be linked to low selenium.
• Poor coat and skin condition: A dull coat, dry skin, or slow hair growth may indicate deficiency.
• Tying up: While tying up has multiple causes, selenium and vitamin E deficiency can be contributing factors.
• Reduced fertility: Mares may have difficulty conceiving or maintaining pregnancy. Stallions may show reduced sperm quality.
• General poor performance: Sometimes the only sign is that the horse just isn't thriving as well as expected.

In Foals

Foals born to selenium-deficient mares are particularly at risk. White muscle disease can affect skeletal muscles (making foals weak and unable to stand or suckle properly) or cardiac muscle (which can be fatal). This is one reason why selenium status in broodmares deserves particular attention.

How to Confirm Selenium Deficiency

The most reliable way to assess your horse's selenium status is through a blood test. Your vet can measure:

• Whole blood selenium: This reflects longer-term selenium status over the preceding weeks and is generally considered the most useful measure.
• Serum selenium: This reflects more recent intake but can fluctuate day to day.
• Glutathione peroxidase (GSH-Px) activity: This enzyme depends on selenium, so its activity in blood gives a functional indication of selenium status.

If you suspect deficiency, a blood test is always worth doing before you start supplementing — partly to confirm the problem and partly because selenium has a narrow margin of safety, and you need to supplement appropriately.

Addressing Selenium Deficiency: Getting the Balance Right

Selenium is one of those minerals where the difference between "not enough", "just right", and "too much" is relatively small. The toxic threshold is only about 5–10 times the requirement — much narrower than for most minerals. This means supplementation needs to be thoughtful and precise.

Sources of Supplementary Selenium

There are two main forms of selenium used in horse supplements and feeds:

• Sodium selenite (inorganic): This is the most common form added to compound feeds. It's effective and well-researched but has a narrower margin of safety.
• Selenium yeast / organic selenium (selenomethionine): This form is generally better absorbed and stored in the body. It's increasingly used in premium supplements and balancers.

Practical Approaches

1. Feed a balancer: If your horse doesn't receive the recommended amount of compound feed, a feed balancer is the simplest way to fill mineral gaps including selenium. Most balancers are designed to be fed at 100–200g per day and provide a full spectrum of vitamins and minerals.

1. Use a targeted supplement: Broad-spectrum vitamin and mineral supplements designed for UK horses typically include selenium at appropriate levels.

1. Selenium-specific supplements: These exist but should be used with caution. It's easy to over-supplement if you're also feeding a balancer or fortified feed.

1. Veterinary intervention: In cases of confirmed severe deficiency, your vet may recommend an injectable selenium and vitamin E preparation (such as Dystosel) to rapidly restore levels.

The Vitamin E Connection

Selenium and vitamin E work synergistically as antioxidants. A deficiency in one places greater demand on the other. In practice, many selenium-deficient horses are also low in vitamin E — especially those on conserved forage (hay or haylage) rather than fresh grass, since vitamin E degrades significantly during the drying and storage process.

When addressing selenium deficiency, it's always worth considering vitamin E status at the same time.

Can You Give Too Much Selenium?

Yes — and this is an important point. Selenium toxicity (selenosis) is a real risk if supplementation is excessive or if multiple selenium sources are fed unknowingly.

Signs of chronic selenium toxicity include:

• Hair loss, particularly from the mane and tail
• Cracking and separation of the hooves
• Lameness
• Weight loss
• In severe cases, organ damage

The maximum tolerable concentration in the total diet is generally considered to be around 5mg/kg dry matter. Toxicity is very unlikely from UK forage alone, but it can occur when multiple supplements or feeds containing selenium are stacked on top of each other.

This is another reason why understanding the full picture of your horse's diet matters. If you're feeding a balancer, a supplement, and a fortified feed, you may be providing more selenium than you realise.

Regional Variation Within the UK

While UK soils are generally low in selenium, there is some regional variation:

• South-west England and Wales: Particularly low selenium soils, often acidic and leached by high rainfall.
• Scotland and northern England: Similarly low, with acidic peaty soils compounding the problem.
• Parts of eastern England: Slightly better selenium status in some chalky or limestone-derived soils, but still well below levels seen in selenium-rich parts of the world.
• Ireland: Generally very low, with similar geological and climatic factors to western Britain.

Regardless of where you are in the UK, it's safest to assume that your horse's forage is not providing adequate selenium unless you have specific evidence to the contrary.

Key Takeaways

• Selenium deficiency is extremely common in UK horses because British soils are naturally low in the mineral, and forage grown on these soils reflects that.
• Most UK horses need supplementary selenium — either through a properly fed compound feed, a balancer, or a targeted supplement.
• Signs of deficiency can be subtle — from poor coat condition and stiffness to immune problems and reduced performance.
• Blood testing is the best way to confirm deficiency and guide appropriate supplementation.
• Selenium has a narrow safety margin — always account for all dietary sources to avoid accidental over-supplementation.
• Vitamin E and selenium work together — consider both when assessing your horse's antioxidant status.

If you're unsure whether your horse's diet is providing enough selenium — or any other essential mineral — taking the time to properly assess what's going in is the single most useful step you can take. It removes the guesswork and lets you supplement with confidence rather than crossing your fingers and hoping for the best.